Relationship of Phthalates and BPA on Thyroid Problems

Summary
Thyroid hormones play an important role in the metabolic functions associated with the cardiovascular, nervous, pulmonary and reproductive systems of the body. Several human and animal studies have shown that thyroid functions may be altered or affected by exposure to phthalates and related compounds in the environment. This study analyzed urine samples from a set of adults and adolescents, participating in the NHANES 2007-08 survey to verify the link between phthalates, bisphenol A (BPA) and thyroid hormone levels. Higher exposure to phthalates, especially DEHP, and BPA was found to be associated with lower levels of thyroid hormones in blood.

Introduction
The critical role of thyroid hormones in human physiology has been well established across an entire spectrum of activities, ranging from fetal maturity, brain development and basic metabolism in nervous, cardiac and other systems in the body. The negative impact of common environmental chemicals such as phthalates and BPA, present in cosmetic products, plasticizers, adhesives, and solvents on thyroid function, has been a matter of concern for a long time. Retardation in brain development, increase in body weight, insulin resistance and diabetes have been linked to phthalates. This study thus aimed to verify the effect of these chemicals on the thyroid function.

Results/Key findings
* In adults, exposure to phthalates and BPA was found to inversely affect thyroid function.
* Total T4 reduced by 4.1 percent and 1.7 percent with an increase in MEHHP and urinary BPA, respectively.
* Results varied with age, gender, body weight and race.
* High levels of the metabolites of DEHP, which is a plasticizer used in manufacturing of articles made of PVC and used in things like glowsticks, were found to have the strongest negative effect on thyroid function.

Next steps/Shortcomings
The study design did not contribute to the understanding of why BPA, phthalate and thyroid functions were related. A single urine sample per subject at a single time may have been inadequate for a thorough analysis. In addition, the serum samples were collected only once and follow up studies were not conducted on the participants.

Conclusion
Evidence from recent studies points to the possible role of DEHP as a thyroid receptor antagonist that modifies the sodium-iodide balance. The authors point out that results obtained in this study indicate a direct effect of phthalates on thyroid hormone synthesis, metabolism, release and transport, rather than on the hypothalamus or the pituitary gland. The results obtained from adults were contrary to those obtained from adolescents in the 12 to 19 age groups. In fact, a positive association was found between DEHP and levels of thyroid hormones in young participants. The reason for this inconsistency will have to be explored in future studies. Hence, authors write, “More detailed studies are needed to determine the temporal relationships and potential clinical and public health implications of these associations.”